1. Field of the Invention
This invention relates to a light-emitting module that emits lights received from a semiconductor light-emitting element such as a semiconductor laser element or the like via a lens, and more particularly, to a technique of monitoring an optical output from the semiconductor light-emitting element.
2. Description of the Related Art
A vertical cavity surface emitting laser diode (hereinafter referred to as VCSEL) is a laser diode that emits lights from a plane of a semiconductor substrate. VCSEL has several characteristics: VCSEL is lower in the drive current than that of an edge-emitting laser diode, the characteristic test can be performed at a wafer level, and can be readily produced into a two-dimensional array. Therefore, VCSEL is in use as a light source for optical information processing or optical communication, or as a light source of a data memory apparatus that stores data by means of light.
Generally, VCSEL is used with hermetically sealed in a package according to the standard TO-CAN packaging or the like. The package includes a lead pin for external connection. A drive signal applied to the lead pin is converted into an optical signal, and a laser beam is emitted from the package. Such emitted optical output varies depending on the temperature or the like. Hence, the optical output of VCSEL is monitored, and the drive current is controlled according to the monitoring result.
Japanese Patent Application Publication No. 2001-343559 (hereinafter, referred to as Document 1), as illustrated in FIG. 16, describes an optical module that includes, on a substrate 1, a light-emitting element 3, an optical fiber 5 optically coupled to the light-emitting element 3, a light-receiving element 4 that monitors the light emitted from the light-emitting element 3. A partially transmitting mirror 8 is also included to partially reflect the light emitted from the light-emitting element 3 and transmitted in the optical fiber 5 across a core axis 5a of the optical fiber 5 in non-parallel directions relative to the core axis 5a, and the light-receiving element 4 monitors the light reflected from the partially transmitting mirror 8.
A light-emitting module 10 described in Japanese Patent Application Publication No. 2004-95824 (hereinafter, referred to as Document 2), as illustrated in FIG. 17, includes a mount element 20 that mounts a semiconductor light-emitting element 21 and a semiconductor light-receiving element 22 such as a VCSEL or the like, a lens 32, and a lens-holding member 30. The light emitted from the semiconductor light-emitting element 21 is partially reflected by a reflection film 32c provided on a first plane 32a of the lens 32, and enters into the semiconductor light-receiving element 22. This makes it possible to receive front surface lights emitted from the semiconductor light-emitting element with the semiconductor light-receiving element without a half mirror.
Japanese Patent Application Publication No. 11-274650 (hereinafter, referred to as Document 3), as illustrated in FIG. 18A, a VCSEL 19 is arranged to face a glass window 8. A photo diode 2 mounted on a PD submount 4 is, as illustrated in FIG. 18B, includes a light-receiving plane having a shape of ring in which a through hole 17 is formed in the center thereof. The through hole 17 formed in the photo diode 2 is provided to partially pass only the light that includes a light axis of a mainly emitted light 12 of the laser diode 19, and only the light that passes through the through hole 17 is externally emitted through the glass 8. A higher-order light emitted at a high angle out of the mainly emitted light 12 is radiated by the photo diode 2 having a shape of ring, and is blocked.
As described heretofore, a ball lens is held as a light-emitting module in which the laser beam emitted from VCSEL is coupled to an optical fiber via a lens, thereby realizing an optical coupling with the optical fiber with a comparatively small part number. There is an advantage in that the fabrication cost is low. On the other hand, as shown in Document 2, the semiconductor light-emitting element is disposed on the light axis of the ball lens and the photodiode is arranged in a position away from the semiconductor light-emitting element. When a light amount of the semiconductor light-emitting element is monitored, there is a disadvantage that a use efficiency of the light amount reflected by the ball lens is low. In particular, when the laser beams emitted from the semiconductor light-emitting element are multi-mode ones, the light-emitting intensity is distributed in a doughnut shape. For this reason, unless the photo diode is aligned with high accuracy, the coupling efficiency with the reflected beam will be degraded. If the laser light amount is increased to enter a light amount enough to operate the photo diode, which is not desirable in light of operating life and safety of the semiconductor light-emitting element such as VCSEL or the like. In addition, the monitoring method described in Document 1 has to include the half mirror provided separately from the lens, leading to a problem of increased costs. In the monitoring method described in Document 3, the light-receiving element partially blocks the laser beam. This completely lacks a light-emitting pattern, and this is not desirable in the optical communication.